Plot the "y=" line (make it a solid line for y≤ or y≥, and a dashed line for y< or y>)
The C(x) equation is a parabola with a squared "x" and a positive leading term coefficient, meaning, is a vertical parabola and it opens upwards
the lowest point, or lowest cost value, it reaches, is the U-turn point, or vertex

so, the lowest cost is at
9 buckets of water because If 1/3 is 3 than you just add 6 more buckets which is 9
Answer is 9
A(x)=f(b)-f(a)/b-a
where a=-10 b=10
f(x)= x^2+9x+18
f(a)=(-10)^2-90+18=28
f(b)=208
A(x)=208-28/10+10
A(x)=180/20=9